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9. Merge Sorted Array

easyAsked at Gojek

Merge two sorted arrays into the first one in-place keeping the combined array sorted.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

You are given two integer arrays nums1 and nums2 sorted in non-decreasing order. nums1 has length m+n with the last n slots set to 0 as placeholder. Merge nums2 into nums1 in-place so that nums1 ends up sorted.

Constraints

  • nums1.length == m + n
  • nums2.length == n
  • 0 <= m, n <= 200

Examples

Example 1

Input
nums1 = [1,2,3,0,0,0], m = 3, nums2 = [2,5,6], n = 3
Output
[1,2,2,3,5,6]

Example 2

Input
nums1 = [1], m = 1, nums2 = [], n = 0
Output
[1]

Approaches

1. Concat and sort

Copy nums2 into the tail and call sort. Easy but throws away the sortedness signal.

Time
O((m+n) log(m+n))
Space
O(1)
for (let i = 0; i < n; i++) nums1[m + i] = nums2[i];
nums1.sort((a, b) => a - b);

Tradeoff:

2. Two pointers from the back

Fill from the rightmost slot using two pointers comparing nums1[m-1] and nums2[n-1] so no element gets overwritten before it is moved.

Time
O(m+n)
Space
O(1)
function merge(nums1, m, nums2, n) {
  let i = m - 1, j = n - 1, k = m + n - 1;
  while (j >= 0) {
    if (i >= 0 && nums1[i] > nums2[j]) nums1[k--] = nums1[i--];
    else nums1[k--] = nums2[j--];
  }
}

Tradeoff:

Gojek-specific tips

Gojek often merges ride and food order streams partitioned by region; show you can merge without an extra allocation, since memory matters on the mobile-first apps.

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Output

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